In various settings, the presence of dissolved gases in samples presents challenges in a number of ways. A few examples include microfluidic applications, lab-on-a-chip (LOC) applications, microfluidic oxygen removal/control in capillary electrophoresis, microfluidic cell culturing platforms, oxygen-sensitive reactors, and analyzing liquid samples for the presence of metals or other contaminants.
With respect to testing for metals, one technique for analyzing a sample is Anodic Stripping Coulometry (ASC). ASC involves configuring an electrolytic cell comprising an anode and cathode; applying potential to the electrodes sufficient to cause an analyte(s) of interest—such as copper, mercury, lead, and cadmium to name a few—to deposit upon one of the electrodes; applying potential sufficient to completely “strip” the analyte(s) from the electrode where they deposited; and characterizing or measuring the analytes in association with the current for stripping. However, any dissolved gases in the sample can cause interference because the gases may also be reduced at potentials needed for electrodeposition of the analytes, skewing the results or limiting the electrode life.
ASC and other electrochemical-based analysis systems, including the electrodes and cells which comprise them, are generally scalable to the microscale without significantly compromising their performance. Taking advantage of this feature, microfabricated electrochemical devices commonly serve as sensors for identifying and quantifying the presence of analytes in a liquid sample, and have been employed remotely for such purposes. Such devices are cost-effective to mass produce, they can be set up to perform remotely, and they are configurable to detect a wide variety of analytes.
Even so, to take full advantage of the opportunity, challenges must be overcome. One of these is the need to remove dissolved gases from samples before analysis occurs (i.e., pretreatment). Oxygen is one such gas, but not the only gas. Oxygen is reduced on silver electrodes in highly basic solution by the following reaction sequence: (1) O2+H2O+2e-HO2-+OH—; (2) HO2—½O2+OH—. P. K. Adanuvor, et al., J Electrochem Soc. 135 (1998) 2509-2517.
However, while the electrochemical removal of dissolved oxygen from a sample has been performed, prior approaches also carry downsides. Some rely on the application of a vacuum, which requires a significant amount of power. Others expose the sample to an oxygen scavenger which can alter its metal speciation. Some have used purging with an inert gas such as argon or nitrogen, which is cumbersome, and may change the pH of the sample. These prior approaches require time-consuming, on-site labor to carry out, making automated remote analysis challenging, if not infeasible. Further, none of these approaches is readily adaptable for use in microfluidic or LOC platforms.
Accordingly, there remains a need for systems, methods and apparatus for removing dissolved gases from a sample, which overcome the limitations of known systems, including the need for direct operator intervention and to avoid having the removal process chemically or physically alter the sample. The removal of dissolved gases from a sample is useful in a range of activities, from sample testing to removal of flow-impeding gas bubbles in LOC applications and other settings where the removal or regulation of dissolved gases is desirable.